The operating principle of proton-exchange membrane fuel cells (PEMFC) is based on the conversion of chemical energy into electric energy by catalytic reaction between a fuel, for example, hydrogen, and an oxidizer, for example, oxygen.
Membrane-electrode assemblies (MEA), commonly called cell cores, form the basic elements of PEMFCs. As illustrated in FIG. 1, a MEA is generally formed of a polymer membrane (electrolyte, 3) in contact with a catalytic layer (electrodes, 2) on both sides. The electrodes, respectively called anode and cathode, are thus separated by the electrolyte which is an electronically-insulating proton-conducting medium. Current collectors (1) ensure the electron transfer at the external surface of the electrodes. Further, gas diffusion layers are arranged on either side of the MEA to ensure an electric conduction, the homogeneous distribution of the reactant gases, and the discharge of the produced water.
In the case of proton-exchange membrane fuel cells, the electrolyte generally is a membrane made of a cation-exchange polymer, such as Nafion® (Dupont) or Aquivion® (Solvay).
The fuel used in proton-exchange membrane fuel cells may be a gas, such as hydrogen, or a liquid, such as for example an alcohol, particularly ethanol, methanol, or also ethylene glycol.
The following reactions, given as an example, illustrate the electrochemical reactions occurring at the electrodes in the case where the fuel and the oxidizer respectively are hydrogen and oxygen:Anode: H2→2H++2e−  (1)Cathode: O2+4H++4e−→2H2O  (2)E°anode=0 V/ENH E°cathode=1.23 V/ENH 
In this case, the general reaction thus is the following:H2+½O2→H2O E°eq=E°cathode−E°anode=1.23 V
The electromotive force across the cell thus is 1.23 V in standard conditions.
At the anode, the decomposition of the hydrogen adsorbed on the catalyst generates protons H+ and electrons e−. The protons then cross the polymer membrane before reacting with oxygen at the cathode. The reaction of the protons with oxygen at the cathode results in the forming of water and in the production of heat.
Such electrochemical reactions are kinetically promoted by the presence of a catalyst forming the electrodes. Although a plurality of materials may be used according to the type of reaction and of fuel, platinum is the most efficient for most reactions and fuels.
As already indicated, the catalyst may appear in the form of catalytic layers, which are generally made of platinum nanoparticles supported on carbon clusters. The catalyst may be uniformly deposited by means of a catalytic ink on the membrane surface or on the diffusion layer. The catalytic ink is particularly made of the catalyst supported on carbon (platinum-on-carbon), a carrier fluid, and a proton-conducting polymer. The latter is generally of same nature as the electrolyte.
The proton-conducting polymer mainly plays a role in the cell performances, since it determines the proton conductivity of the cell. It is an ionomer, that is, a polymer having an ionized group allowing a charge transport.
Initially, the ionomers used to be sulfonated phenols, and then polystyrene sulfonic acid polymers, which are more mechanically resistant. Currently, perfluoro sulfonic acid materials (PFSA) are widely used in present fuel cells, due to their good chemical and mechanical resistances.
PFSAs are ionomers derived from perfluorosulfonic acid, that is, comprising sulfonate groups SO3−. They further are fluorinated polymers.
This family of ionomers gathers a number of compounds, which differ by a slightly different chemistry. PFSA materials are commercialized under trade names Nafion® (Dupont), Aquivion® (Solvay), Flemion® (Asahi Glass Company), or Aciplex® (Asahi Chemical Company). As an example, the partial structure of Aquivion® (Solvay) is illustrated in FIG. 2.
As can be observed, these polymers essentially differ by the chemical structure of their side or dangling chains, that is, the chains grafted on the main chain of the polymer (or branchings).
Thus, the side or dangling chains of Aquivion® are shorter (“short side-chains”) than those of Nafion® (“long side-chains”). This translates as a higher crystallinity rate and vitreous transition temperature.
Further, Aquivion® has a lower equivalent mass. Hydrophilic groups are thus by a larger number for a sane mass, which gives it a stronger hydrophilic character. This property provides a better proton conductivity, due to a larger number of sulfonated sites, which improves properties at low humidity.
The chemical characteristics of such PFSA ionomers thus have an impact on the operation of the cells containing them.
Thus, Nafion® provides good performances whatever the relative humidity of the gases. Aquivion®, however, has better performances than Nafion® at a low relative humidity, which may make it advantageous in such conditions. However, the use of Aquivion® as a proton-conducting polymer raises an issue at high humidity, with significant drops in activation performances. Arico et al. (“High temperature operation of a solid polymer electrolyte fuel cell stack based on a new ionomer membrane”, Fuel Cells 10, 2010, No. 06, 1013-1023) have also shown that Aquivion® is better than Nafion® at medium temperatures, in the order of 100-110° C.
As appears from the foregoing, the different currently-available PFSAs each have advantages and drawbacks. Another important parameter capable of hindering their use relates to their cost.
There thus is an obvious need to provide new proton conduction polymers usable in the context of PEMFCs, particularly at the level of their catalytic layers.